Have you been on the Depo shot? I had a mysterious pain much like you are describing about six years ago. I also had a low grade fever with it most of the time, and my white blood cell counts were elevated (at one point to extreme amounts). I would only be able to walk about 20 feet, on a good day, before having to sit down from the pain.
I had tests to check my appendix and my gull bladder. They even checked to see if my ovaries might be to blame. I was in and out of doctors and surgeons for about two months. I was going days at a time without eating because it hurt too bad; but then I realized I was still gaining weight during this time because the Depo was messing with my hormones.
When it was time for my next shot, I decided to switch from the Depo shot to a low-dose pill. Within a few days, the pain was almost completely gone. The fever disappeared just as fast. My white blood cell counts returned to normal. Evidently the Depo shot was reeking havoc on my system.
If you're not on Depo, I suggest looking into other medications you might be taking. One of them could easily be to blame.